The present invention relates to a four wheel drive power transmission system, and more particularly relates to such a four wheel drive power transmission system for a vehicle such as an automobile adapted for four wheel drive operation, in which the construction is improved so as to better provide a variety of operational modes.
The present invention has been described in Japanese Patent Application Ser. No. 60-194635 (1985), filed by an applicant the same as the entity assigned or owed duty of assignment of the present patent application; and the present patent application hereby incorporates into itself by reference the text of said Japanese Application and the claims and the drawings thereof; a copy is appended to the present application.
Further, the present inventors wish hereby to attract the attention of the examining authorities to their copending patent applications Ser. Nos. 06/903,241 and 06/903,239, which may be considered to be material to the examination of the present patent application, and which are coowned with or are subject to obligations of coassignment together with the present patent application.
There are various types of known four wheel drive power transmission systems for automotive vehicles. One such type is the so called "part time" four wheel drive power transmission system, in which a clutch provided in a transfer device for four wheel drive is used to switch the transmission system between a four wheel drive mode and a two wheel drive mode; such a part time four wheel drive power transmission system is, for example, disclosed in Japanese Utility Model Laying Open Publication Ser. No. 58-29519 (1983). On the other hand, another type of four wheel drive power transmission system is the so called "full time" four wheel drive power transmission system, in which a clutch provided in a central differential device for four wheel drive is used to switch said central differential device between a first operational mode in which it provides differential action between the combination of the front wheels of the vehicle and the combination of the rear wheels of the vehicle, and a second operational mode in which it does not provide any such differential action, but instead drives said front wheel combination and said rear wheel combination individually, i.e. by effectively connecting these combinations together. Such a full time four wheel drive power transmission system is, for example, disclosed in Japanese Utility Model Laying Open Publication Ser. No. 47-203 (1972) and also in Japanese Patent Application Ser. No. 53-145857 (1978), which has been laid open to the public as Japanese Patent Laying Open Publication Ser. No. 55-72420 (1980).
Further, there has already been disclosed in the aforesaid Japanese Patent Application Ser. No. 53-145857 (1978), laid open as Japanese Patent Laying Open Publication Ser. No. 47-72420 (1980), a four wheel drive power transmission system having, in addition to a differential clutch which as mentioned above can selectively put the central differential device into a locked state or a free state, also a power transmission control clutch which carries out switchover between two wheel drive operation and four wheel drive operation, and such that the vehicle driving state may be switched over between three types of operational state, to wit: central differential action type four wheel drive (in which the central differential device is operational to provide differential action), central differential non action four wheel drive (in which the central differential device is locked up), and two wheel drive.
The problem which occurs with these prior arts is the following. It is known that generally in a four wheel drive vehicle such as an automobile it is preferable, for straight ahead steering stability during high speed driving, for the vehicle drive to be performed all or largely by the front wheels of the vehicle, with the rear wheels providing only a subsidiary amount of drive or no drive at all. On the other hand, for starting off from rest and for climbing inclines such as hills it is preferable for the vehicle to be performed, contrariwise, all or largely by the rear wheels of the vehicle, with the front wheels providing only a subsidiary amount of drive or no drive at all. And, further, for driving on slippery surfaces such as wet roads, snow, or mud it is desirable to perform four wheel driving with the central differential device set to the locked up state for definitely powering both the front wheels and the rear wheels, even in the case of slippage of one of said wheel pairs.
However, in the above outlined prior art variable drive type four wheel drive systems, since conventionally the cental differential device is a so called balanced distribution type differential device which includes a bevel gear mechanism, such as is conventionally used as a differential device for providing differential action between two coaxial wheels of a conventional automobile, even if the above described differential clutch, which as mentioned above can selectively put the central differential device into a locked state or a free state, and the above described power transmission control clutch, which carries out switchover between two wheel drive operation and four wheel drive operation, are operated independently, nevertheless the drive torque distribution ratio between the front and the rear wheels can only be changed between the two values of 50%-50% and 100%-0% or 0%-100%, and thus, even if one of front wheel emphasis drive and rear wheel emphasis drive can be performed, they cannot both be performed in the case of any one configuration. That is to say, if the power transmission control clutch is provided in the power transmission path to the front wheels of the vehicle, then two wheel driving by means of the rear wheels only can successfully be performed, but front wheel emphasis drive cannot be performed at all; whereas, if power transmission control clutch is provided in the power transmission path to the rear wheels of the vehicle, then two wheel driving by means of the front wheels only can successfully be performed, but rear wheel emphasis drive cannot be performed at all.